DataContext of usercontrol in WPF - wpf

Question

I'm new to WPF and I'm trying to start a little project with a maximum of good practice. I'm using MVVM and dependency injection.
I have a concern which seems to be easy to understand but i can't find an answer (at this step, DataContext is not very clear for me).
The UserControlView of type UserControl contains just a button for testing.
This is the app class :
public App()
{
IServiceCollection services = new ServiceCollection();
services.AddSingleton<MainWindow>();
services.AddSingleton<UserControlViewModel>();
services.AddSingleton<UserControlView>();
_serviceProvider = services.BuildServiceProvider();
}
The user control is included in the Main windows like that :
<Grid>
<views:UserControlView/>
</Grid>
Now, in the OnStartup overrided method :
protected override void OnStartup(StartupEventArgs e)
{
MainWindow = _serviceProvider.GetRequiredService<MainWindow>();
MainWindow.DataContext = _serviceProvider.GetRequiredService<PaymentMeansViewModel>();
MainWindow.Show();
}
Like that it works, my button is correctly binded to the command.
But what is strange for me is that I have to set the 'UserControlViewModel' as the DataContext of the Main Window.
Isn'it possible to bind it to the 'UserControlView', something like :
protected override void OnStartup(StartupEventArgs e)
{
MainWindow = _serviceProvider.GetRequiredService<MainWindow>();
UserControlView testUC = _serviceProvider.GetRequiredService<UserControlView>();
testUC.DataContext = _serviceProvider.GetRequiredService<UserControlViewModel>();
MainWindow.Show();
}
Thanks for help.

Answer 1

Finally I did it.
I think (I hope I'm right) that I understood.
First of all, let's begin with the basic.
A view must have a viewmodel to bind the properties. A usercontrol is a kind of view "encapsulated" in a view. Therefore a usercontrol must have its own viewmodel and the view must have its own viewmodel.
The datacontext of the MainWindow is set in the app onstartup method :
MainWindow = new MainWindow()
{
DataContext = new MainWindowViewModel()
};
MainWindow must implement INotifyPropertyChanged. All view models must implement this interface. We can create a base class which will be derived in the view models :
public class ViewModelBase : INotifyPropertyChanged
{
public event PropertyChangedEventHandler? PropertyChanged;
protected void OnPropertyChanged(string? propertyName)
{
PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
}
}
The DataContext of the usercontrol must be explicit in the xaml of the MainWindow:
<Grid>
<views:UserControlView DataContext="{Binding CurrentViewModel}"/>
</Grid>
"CurrentViewModel" is a DataContext, then it's a ViewModel, and as it is binded, it must be a property of the MainViewModel.
public class MainWindowViewModel : ViewModelBase
{
public ViewModelBase CurrentViewModel { get; }
public MainWindowViewModel()
{
CurrentViewModel=new UserControlViewModel();
}
}
Hope it can help.